The present invention relates to a gradient magnetic field application method and apparatus and a magnetic resonance imaging apparatus, and more particularly to a method and apparatus for applying a gradient magnetic field in exciting spins of atomic nuclei, and a magnetic resonance imaging apparatus employing such a gradient magnetic field application apparatus.
In magnetic resonance imaging, magnetic field gradients are generated in a space to be imaged to allow the three-dimensional position of voxels to be identified by frequency of magnetic resonance signals generated by spins of atomic nuclei, such as protons. The generation of the magnetic field gradients involves generating a static magnetic field a having a uniform magnetic field strength B0 in a field of view (FOV), as exemplarily shown as a profile of the magnetic field strength in FIG. 1, applying a symmetric gradient magnetic field b having a direction on one side and a direction on the other side opposite to each other with respect to the center O of the FOV, and obtaining a composite magnetic field c with a gradient by composing the magnetic fields a and b. To generate the static magnetic field a, a superconductive electromagnet, a normal conductive electromagnet, a permanent magnet or the like is employed. To generate the gradient magnetic field b, a gradient coil is employed having an appropriate loop shape.
When such a gradient magnetic field is generated, a peripheral region d is formed outside the FOV in the left portion of the drawing, i.e., on the side on which the static magnetic field a and the gradient magnetic field b have the same direction, where the composite magnetic field c has the, same strength as the magnetic field within the FOV, because both the strengths of the static magnetic field a generated by the magnet, and the gradient magnetic field b generated by the gradient coil decrease on both sides of the FOV. Accordingly, the magnetic resonance signals generated in the peripheral region d have a frequency common to the magnetic resonance signals generated in the FOV, resulting in artifacts in the obtained image due to wraparound from the outside of FOV.
Magnetic resonance imaging employing spin echoes attempts to reduce such artifacts by making the strength of a gradient magnetic field G90 applied in 90xc2x0 excitation and the strength of a gradient magnetic field G180 applied in 180xc2x0 excitation different, as exemplarily shown in FIG. 2. Then, the peripheral region d90 by the gradient magnetic field G90 and the peripheral region d180 by the gradient magnetic field G180 become different from each other, and those spin echoes which induce artifacts are generated in a reduced region dat where these peripheral regions overlap.
However, although the region in which the artifact-inducing spin echoes are generated is reduced by the above technique, it does not completely disappear. Therefore, the effect of reducing artifacts is still insufficient.
It is therefore an object of the present invention to provide a gradient magnetic field application method and apparatus for preventing artifacts due to a magnetic field outside a field of view (FOV), and a magnetic resonance imaging apparatus employing such a gradient magnetic field application apparatus.
In accordance with a first aspect of the invention, there is provided a gradient magnetic field application method for applying a gradient magnetic field in performing a plurality of RF excitations of spins of atomic nuclei, the method comprising the steps of making at least the polarity of the gradient magnetic field in a first RF excitation and the polarity of the gradient magnetic field in the next RF excitation opposite to each other.
In accordance with a second aspect of the invention, there is provided a gradient magnetic field application apparatus for applying a gradient magnetic field in performing a plurality of RF excitations of spins of atomic nuclei, the apparatus making at least the polarity of the gradient magnetic field in a first RF excitation and the polarity of the gradient magnetic field in the next RF excitation opposite to each other.
In accordance with a third aspect of the invention, there is provided a magnetic resonance imaging apparatus for performing a plurality of RF excitations of spins of atomic nuclei within a subject to be imaged in the presence of a gradient magnetic field, and producing an image based on magnetic resonance signals generated by the spins, the apparatus comprising gradient magnetic field application means for making at least the polarity of the gradient magnetic field in a first RF excitation and the polarity of the gradient magnetic field in the next RF excitation opposite to each other.
In accordance with a fourth aspect of the invention, there is provided a magnetic resonance imaging method for performing a plurality of RF excitations of spins of atomic nuclei within a subject to be imaged in the presence of a gradient magnetic field, and producing an image based on magnetic resonance signals generated by the spins, the method comprising the steps of making at least the polarity of the gradient magnetic field in a first RF excitation and the polarity of the gradient magnetic field in the next RF excitation opposite to each other.
According to the present invention, a region in which spin echoes are generated outside the FOV is removed by making at least the polarity of a gradient magnetic field in a first RF excitation and the polarity of the gradient magnetic field in the next RF excitation opposite to each other.
Thus, the present invention can provide a gradient magnetic field application method and apparatus for preventing artifacts due to a magnetic field outside a field of view, and a magnetic resonance imaging apparatus employing such a gradient magnetic field application apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.